Thanks for all your suggestions.
The displays are red.
I drove individual segments on a breadboard to start with for testing purposes using the 3.3v supply from an Arduino Uno via a 22 ohm resistor and measured 3.2mA per segment.
I use the code on other projects so I will set to checking my wiring...
IIRC, 3.2mA is somewhat high for a pi's GPIO to provide - for single LEDs a series resistor of ~220 - 560 ohms (depending on LED colour) is more common (associated with a current of ~1mA). Hence my use of 3.3V to 5.0V level shifters and 5V I.C.'s capable of providing greater (or controlled, limited) currents. FWIW, there are other example LED display circuits within my webpages (as well as the "early" one I linked to).
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm